The present invention relates to a starting device of a spark-ignition multi-cylinder engine.
For example, JP2004-301078A discloses an engine starting device for automatically starting (i.e., restarting), when an engine has automatically stopped since a predetermined stopping condition is satisfied, the engine when a predetermined starting condition is satisfied. In order to promptly start the engine, the starting device firstly performs injections of fuel and ignitions inside cylinders which are respectively on expansion stroke and compression stroke at an engine stopped timing, so as to rotate the engine in a normal rotating direction. Subsequently, the starting device also performs injections of fuel and ignitions inside cylinders which are respectively on intake stroke and exhaust stroke at the engine stopped timing and will be on the compression stroke after the respective strokes. However, when restarting the engine, since a temperature of the engine is comparatively high, a temperature of air inside the cylinder on the intake stroke at the engine stopped timing becomes comparatively high. Therefore, the temperature inside the cylinder increases to be extremely high as the compression of the air inside the cylinder progresses on the compression stroke, and the fuel injected into the cylinder may pre-ignite near a compression top dead center (CTDC). Thus, the starting device in JP2004-301078A prevents such pre-ignition by controlling a fuel injection into the cylinder which is on the intake stroke at the engine stopped timing, in a manner that the fuel injection thereto is prohibited or, during the compression stroke, retarded more than a normal timing.
Moreover, JP2009-041460A discloses an engine automatic starting device which divides fuel to be injected and injects, during intake stroke, the divided fuel into a cylinder which is on the intake stroke at an engine stopped timing, so as to improve homogeneity of the fuel. Thus, the automatic starting device prevents pre-ignition similarly to JP2004-301078A.
Note that, pre-ignition produced in an engine start causes a disadvantage of generating vibration.
Meanwhile, there are engines each having a valve phase variable mechanism for changing the open and close timings of an intake valve. With such an engine, especially when the valve phase variable mechanism is driven by a hydraulic pressure supplied from an engine-driven hydraulic pressure supply source, a predetermined hydraulic pressure cannot be obtained at least in an early stage of the engine start. Therefore, the intake valve is locked to close at a predetermined timing. Note that, the phrase “engine start” herein includes a force start due to a key-on operation by a driver, in addition to a restart after the engine is automatically stopped because a predetermined stopping condition is satisfied. Specifically, in order to achieve a cold-start of the engine, the locked timing of the intake valve may be set to correspond to a close timing which achieves a comparatively high effective compression ratio, in other words, it may be set so that the intake valve is closed at a timing corresponding relatively near to an intake bottom dead center (IBDC) of compression stroke. Note that, the effective compression ratio is a ratio between an in-cylinder volume when the intake valve is closed and an in-cylinder volume when a piston of the cylinder is at the TDC (i.e., a volume of a combustion chamber).
However, in the case where the locked timing of the hydraulic valve phase variable mechanism is set to have the comparatively high effective compression ratio, if the engine start is attempted with a comparatively high engine temperature, a gas temperature and pressure in the end of the compression stroke increase due to an increased in-cylinder temperature and the comparatively high effective compression ratio described above, resulting in a disadvantage of easily causing pre-ignition.
In this regard, as disclosed in JP2004-301078A and JP2009-041460A, devising the mode of the fuel injection into the cylinder is, although effective in avoiding pre-ignition to some extent, but causes degradation in the engine starting performance because the torque in starting the engine may decrease. Particularly, when restarting the engine after the automatic stop, pre-ignition easily occurs on one hand due to the comparatively high engine temperature and the comparatively high effective compression ratio, while a prompt engine start is required on the other hand.